Line pulse keyed automatic gain control circuit with control voltage delay



July 3, 1951 H, A,

LINE PULSE KEYED BASS AUTOMATIC GAIN CONTROL CIRCUIT WITH CONTROL VOLTAGE DELAY Filed Aug. 1,1949 3 Sheets-Sheet l Es @Q m 9 4H1 IN V EN TOR. HARLAND A. BASS ATTORNEYS July 3, 1951 H BASS 2,559,03

LINE PULSE KEYED AUTOMATIC GAIN CONTROL CIRCUIT WITH CONTROL VOLTAGE DELAY Filed Aug. 1 1949 5 Sheets-Sheet 2 APPL IEO TO v, APPLIED 7'0 721655 v APPLIED 70 RF. STAGE APPL IED AGO POTENTIAL INVENTOR. HARLAND A. BASS ATTOR/V YS y 3, 1951 H. A. BASS 2,559,038

LINE PULSE KEYED AUTOMATIC GAIN CONTROL CIRCUIT WITH CONTROL VOLTAGE DELAY Filed Aug. 1, 1949 3 Sheets-Sheet 5 PEAK-PEAK VOL rs IN VEN TOR. HARLAND A BASS Patented July 3, 1951 LINE PULSE KEYED AUTOMATIC GAIN CONTROL CIRCUIT WITH CONTROL VOLTAGE DELAY Harland A. Bass, Mount Healthy, Ohio, assignor to Avco Manufacturing Corporation, Cincinnati, Ohio, a corporation of Delaware Application August 1, 1949, Serial No. 107,962

1 Claim. 1

The present invention is a novel automatic gain control (A. G. C.) circuit for television receivers, specifically a gain control circuit which is adapted to apply control potentials both to the radio frequency (R. F.) input stage and to one or more intermediate frequency (I. F.) stages in such -'a manner that such control potential is first applied to the I. F. stages as the video signal level increases and later to the R. F. input stage.

A number of automatic gain control circuits have been developed, and research in this 'field is very active. It has been directed to the find- .ing of an efficient gain control circuit with good brightness, stability, freedom from flicker normally caused by line Voltage fluctuations, a high degree of immunity from noise interference and ,easy adaptability for use in conventional television receivers, with a minimum loss in gain and band width and a minimum .cost. The present invention, while directed generally to those objectives, is addressed specifically to the objective of automatically providing a diversity or delay between the gain control potentials applied to the radio frequency input stage and the controlled -I. F. stages, in such a manner as to assure a reconciliation of the conflicting require ments of desired high signal-to-noise ratio and maximum sensitivity and the avoidance of undesired overload with strong signals. In other words, the circuit in accordance with the present "invention is soarranged that a portion of the available control potential is applied only to the I. F. stages at low signal input level, and that full control potential is applied to both I. F. and R. Fcstages at high signal input levels.

If any application of A. G. C. potentials to both R. F. and I. F. stages is delayed until a good signal-.to-noise ratio is attained, an overload condition can occur in the I. F. amplifier stages. On the other hand, if control potential is applied to the I. F. stages without any delay, then there is a sacrifice of sensitivity.

It is an object of the present invention to reconcile the conflicting requirements for an ideal A. G. C. system by providing an arrangement which performs these functions:

'(1) It delays the application of full control potential to either I. F. or R. F. stages until the output of the video detector has reached an adequate level;

(2) It delays the application of any gain con- 'trol potential to the R. F. stage until the carrier input level is such as to insure a satisfactory signal-to-noise ratio at the input of the I. F. amplifier;

(3) It .applies partial control potential to the I. F. stages at low signal input level before any A. G. C. potential is applied to the R. F. stage,

to insure that no overload develops on strong signals.

Fora better understanding of the present invention, together with further objects, advantages, and capabilities thereof, reference is made to the following description of the accompanying drawings, in which there is illustrated a preferred illustrative form of A. G. C, system in accordance with the invention, as embodied in a television receiver.

Fig. -1 of the-drawings is a circuit diagram,;partly in block form, of a television receiver including my novel A. G. C. circuit; Fig. 2 is a set'of graphs used as an aid in describing the operation of the invention; and Fig. 3 is a set of performance curves showing how the developed A. G. C. potentials at the 'I. F. and R. stages vary with signal input to the antenna circuit.

In accordance with a narrow aspect of the invention, there is provided an automatic gain control system, for use with a television receiver of the type including a gain-controlled "video channel having radio frequency and'intermediate frequency stages, a video detector and a line-frequency deflection system providing a source of line-frequency pulses. This system comprises the combination of means I35, etc. keyed by the line-frequency signal output of said detector '48, etc. for providing a source of negative potential having an amplitude which varies in accordance with the detector output, a source of positivepotential +B, aresistor I88, I8], I82 connected between said potential sources, a diode I43 coupled between a point Y on said resistor and ground and having its anode connected to said resistor for holding said point Y substantially at ground potential until the output'of said detector reaches such a level that said point Ygoes negative, means I83, I88, I84, I86, I85 for coupling said point to said radio frequency stage 2, etc. to apply an A. G. C. potential thereto, and means I54, I3, etc. for coupling another point J on said resistor to said intermediate frequency stage to apply an A. G. C. potential .thereto,said other .point J being located between the first-mentioned point Y and the source of negative potential, whereby a portion of the available negative A. G. Qpotential is first applied to the I..F..stags alone, and, after an increase in carrier signal level, A. G. C. potentials not affected by said diode are applied to both R. F. and I. F. stages.

In accordance With a broad aspect of the invention, there are provided, in combination, a source of increasing negative voltage I38, I40, I35, etc., a resistor I80, I8I, I82 having one terminal connected to said source, means for maintaining a point Y on said resistor at ground potential until the negative voltage output of said Work 4|, 42, 43.

source attains a predetermined value, said means comprising a source of positive potential +B connected to the other terminal of said resistor and a rectifier I ls-connected between said point Y and ground, whereby other points (such as J) on said resistance between said point and said negative source swing increasingly negative relative to the first-mentioned point as said negative voltage increases and whereby said point Y finally swings negative as said negative voltage becomes sufficiently great, whereupon said rectifier becomes non-conductive and at least one point (between X and Y) on said resistor between said rectifier and said positive voltage source swings negative.

The specific description begins with a brief explanation of a typical channel which is controlled by the automatic gain control circuit in accordance with the invention. In the present illustrative disclosure, the novel automatic gain control circuit is shown as incorporated in a superheterodyne television receiver of the intercarrier sound type. This receiver includes the usual antenna, a cathode input radio frequency stage including a triode 2, having an inductor 3 between its cathode and ground. The anode of tube 2 is conof an oscillator modulator stage 5, the latter being shown in block form. The other input termithe block 5 is an anode resistor I which is connected through line 8 to a plate supply filter net- The oscillator modulator stage also includes a pair of output terminals 9 and II] which are coupled to the input of the first intermediate frequency stage.

nected to a terminal 4, one of the input terminals The remaining circuit elements within the oscillator modulator stage indicated by the block 5 are not shown in detail. A Mallory inductuner may be employed in lieu of the elements designated 2, 3, 4, 5, B, I, 8, 9, and I0, such tuner be-, ing well-known to those skilled in the art and fully described in the following publications:

Photofact Television Course, 1949, Howard W. Sams 8: Co., Inc., Indianapolis '7, Indianze-particularly pages 151 and 152; Crosley Television Information Bulletin (No. 368, copyright 1948,

Crosley Division, Avco Manufacturing Corporation, Cincinnati 25, Ohio; also Crosley Service Information Bulletins Nos. 385, 3'76, and 3 80.

p The intermediate frequency video carrier signals in this receiver are applied through a coupling capacitor II to the input circuit of an I. F.

Jamplifier tube 12, the grid of this tube being returned to an automatic gain control line through a resistor l4, the cathode of this tube being connected to a biasing resistor l9. This first ,I. F. stage is coupled, as by an inductor l6 and a capacitor ii, to the input circuit of a sec- ,ond I. F. stage including an electron tube l8,

which is similarly provided with a grid resistor I3 and a cathode impedance 2!] and is coupled to the input of a third I. F. stage as by an inductor 2| and a capacitor 22. The third-I. F.

stage comprises an electron tube 23, the cathode of which is biased by a resistor 24, by-passed by a capacitor 25. This stage is similarly coupled,

' as by an inductor 26 and a capacitor 21, to the input of a fourth and final I. F. stage consisting of an electron tube 28, the cathode of which is connected to a biasing resistor 29, by-passed bya capacitor 30. Plate and screen potentials for these tubes are'furnished from a source of space currenttnot hown) coupled to a filter network ductor 45, a capacitor 46, and an inductor 52, to

a conventional detector network comprising a diode 48 and other circuit elements, the purpose of which isto produce a fiat video response characteristic throughout a wide range of frequencies up to more than four megacycles. Specifically, this detector includes a network consisting of a load resistor 49, a capacitor 58, shunt peaking inductor 5| and series peaking inductor 53, the latter being shunted by a damping resistor 54 and connected to the grid of a single tube 55 included in a video frequency amplifying stage. a A variable cathode bias is provided for-contrast control purposes, degeneration being adjusted as desired. The screen is b y-passed by a capacitor 57, and the anode output is taken off through a type of constant K peaking circuit, comprising a parallel combination of aninductor and a damping resistor Bl, a resonant circuit, comprising a parallel combination of inductor 5B and capacitor 59, an inductor 64 and a resistor 65, and

a coupling capacitor 62 connected to the grid of a cathode ray picture tube 63. The anode of the video amplifier has a load comprising a series combination of an inductor 64 and a-resistor 55, leading to a source of space current not shown. The direct current reinserting circuit comprises a grounded-grid triode 66 having its cathode coupled to the lower end of coil 64 through a capacitor 61 and a resistor 12 and having its plate connected to the lower end of resistor through a resistor 68. The cathode-anode circuit of'triode 68 is shunted by a series combination of resistors 69 and 10. As is well-known to those skilled in the art, a direct current component which is a'measure of the over-all illumination appears at the cathode of tube 66 and is applied to the grid of the picture tube through a resistor H. D. C. restorer networks of this general character are shown and described in a number of publications, including U. S. Patent No. 2,240,281, issued to R. C. Ballard on April 29, 1941.

In the foregoing description, and in fact throughout this disclosure, wave traps, filters, iron cores, suppressor grids, and other elements which need not be shown in order to illustrate or to disclose or describe the present invention or an appropriatebackground for the invention are omitted for purposes of simplicity and clarity, so far as possible. For the same purposes, pentodes are shown in places as triodes.

The above-described elements which begin with the reference numeral 3 and end with the reference numeral 'lz oollectively show the channel which is controlled and the significant element of that channel between the tuner and the picture tube. In brief, these elements show that which is controlled by the A. G. C. system in accordance with the invention. It will be undercussion of the arrangement by which the keying pulses are developed.

The horizontal and vertical synchronizing pulses are applied, as by a conductor I5, from the anode of the grounded grid D. C. restorer and sync separator stage to a conventional synchronizing signal separator I8, herein shown in block form, this unit performing the usual function of separating the vertical pulses from the horizontal pulses. A terminal ll of the separator network 18 is connected to the vertical deflection system, which need not be shown herein for purposes of describing the present invention. The illustrative horizontal system herein shown comprises a blocking oscillator with automatic pulse width control. The horizontal syn chronizing pulses are applied with positive polarity from terminal I8 of the synchronizing signal separator through a coupling capacitor I9 to the grid of a triode 80. High voltage negative pulses obtained from the horizontal deflection system are partially integrated and attenuated by a net-work com-prising series resistor BI, series capacitor 82, and variable shunt capacitor 83, and are also applied to the grid of tube 80. Pulses of a third wave shape are obtained from the discharge capacitor 84 and integrated by a network comprising series resistor 85 and shunt capacitor 83 to form parabolic wave shapes. Tube 80 is the control tube and is biased near cut-off by the D. C. component of the blocking oscillator grid voltage applied through resistors 86 and 8". The plate current of tube 80 consists essentially of pulses the width of which is deter mined by the relative phases of the synchronizing pulses from terminal I8 and the parabolic pulses formed by the network 83,85. The voltage developed across a resistor 88 by this average plate current is injected from the cathode circuit of tube 80 into the grid-of the blocking oscillator tricde 89 by way ofa resistor 90, in order to main-- tain thephase relationship between the oscillator output and the synchronizing signals. The catho'cle circuit of tube 80 is an integrating network comprising resistors 88, 9 I, and 92 and capacitors 93 and 94. The cathode circuit integrating ne work has a fast response, in that capacitor 94 is relatively small, and a slow response, in that resistors 9| and 88 are relatively large. The fast time-constant network tends to prevent hunting, while the slow time-constant network filters out disturbances of greater duration.

The plate of the control tube is by-passed by a capacitor 95 and connected to the anode voltage line 96 through a potentiometer 91.

The automatic pulse width control circuit is "fully described in the article entitled Automatic Frequency Phase Control of Television Sweep Circuits, Proceedings of the I. R. E., 1949, pages 497 et seq., volume 3'7, No. 5, published by the Institute of Radio Engineers, New York, May 1949, and also at page 84 of the above-mentioned Photofact Television Course publication. Refer- Jence is made to these publications for a complete description of the automatic pulse width control circuit. a

. The plate current flow of tube 80, passing through the cathode resistor 88, controls the grid circuit time constant of the blocking oscillator tube 89 to produce synchronization, this resistor 88. being com-monzto the ,grid circuit of blocking oscillator tube 89 and the time-constant circuit comprising capacitor 99, resistor 90, and resistor 88.

.The. blocking oscillator circuit comprises a triode 89, an auto-transformer or inductor I ar- :ranged in a series combination with capacitor 99 coupled between plate and grid, and a dis charge capacitor 84- efiectively coupled between a tap I8I- on the auto-transformer and ground. A resonant circuit comprising a parallel combination of an inductor I02, a capacitor I03, and a damping resistor HM is interposed between this tap and the high potential terminal of capacitor 84. Plate voltage is supplied to the blocking oscillator through a circuit comprising lead 96, dropping resistor I05, inductor I02, tap NH, and a part of auto-transformer I00. The sawtooth voltages employed for horizontal deflection are developed across discharge capacitor 84, tube 89 functioning not only as a blocking oscillator tube but also as a discharge tube, as described in the afore-mentioned article in the May 1949 issue of Proceedings of the I. R. E.

The discharge capacitor is coupled, as by a capacitor I08 and a grid resistor I07, to the grid of a horizontal output amplifier tube I08, the latter being provided with a cathode resistor I09 b-y-passed by a capacitor I I0. The output of this amplifier stage is coupled by a tranformer network IIZ-I I5 to the deflecting coils III and the current waves appearing in the plate circuit of this amplifier tube are employed to produce periodically recurring sawtooth currents of line frequency in coils III, thereby to deflect the electronic beam in the picture tube at line frequency.

The system intercoupling the horizontal output tube and the deflection yoke will be understood by reference to the following patents and publications: Kiver, Television Simplifier, pages 207-213, second edition, 1948, D. Van Nostrand, Inc., New York; U. S. Patent No. 2,910,418, Tourshou. Reference to those publications is made for a detailed description of this network. Briefly, however, the primary II2 of the horizontal output transformer is coupled to a secondary II3 comprising series portions IM and 5, of which portion I I5 is coupled across the yoke circuit l! I, H8. It will be understood that the coils III form part of a yoke assembly encircling the neck of the cathode ray image-re producing tube 83.

The voltage variations applied to the control electrode of power tube I08 produce arising current in tube I08 during scansion, which current is cut ofi at the beginning of retrace time. The current in the deflection coils III and the horizontal output transformer does not disappear at the instant of cut-off of tube I08, however, due to the inherent distributed capacity of the circuit. The inductance of these coils and the transformer, together with the above-mentioned distributed capacity, forms a tuned circuit in which high frequency oscillations would normally be produced. These oscillations begin with the start of retrace time and continue for one-half of the normal period of oscillation, the oscillation being stopped at the negative current peak by a series combination of a diode H8 and a capacitor II9 connected across the secondary H3. The polarities immediately following re trace are such that damper tube I I8 conducts and continues to conduct until tube I08 again becomes conductive. The voltage developed across capacitor II 9 is such as to increase or boost the voltage of the D. C. power source (not shown) connected to terminal I20. It willbe noted that anode potential line 96 is connected to this terminal through a series circuit comprising resistor I2I, capacitor H9, and line I22, to supply anode potential to tubes and.

The plate of tube I08 is connected to terminal I20 through primary H2, inductor I23, capacitor H9, and lead I22, to supply anode potential to tube I08.

The primary winding H2 of the horizontal output transformer is connected to a capacitor I24 in such a manner that a varying voltage is developed across capacitor I24, which voltage is applied to capacitor I I9 through an inductor I23, for purposes of linearity control. For purposes of width control, there is connected in shunt with portion H4 of secondary H3 an inductor I25. The operation of the width-control is well-known to those skilled in the art and is considered in such publications as U. S. Patent 2,449,969 to Antony Wright.

The horizontal output transformer also includes a winding I29 having two portions, I30 and I3I, and a tap I32, the Winding I29 being inductively coupled to the primary and secondary windings.

The elements beginning with the referenc numeral I and ending with the reference numeral I32 are herein shown for the purpose of disclosing an illustrative source of keying pulses employed in the automatic gain control system in accordance with the invention. It will be understood that the keying pulse source collectively designated by the reference numerals beginning with and ending with I32 is illustrative and that other arrangements for deriving keying pulses from the horizontal deflecting system may be employed.

Having described the channel which is controlled and the circuit arrangements which function as a source of keying pulses for the automatic gain control system in accordance with the present invention, the description now proceeds to the latter.

The source of negative potential employed in the automatic gain control circuit in accordance with the invention comprises a keyed pulse rectifier tube I35. This tube is periodically rendered conductive, at line-frequency rate, by pulses derived, from the horizontal deflection system. Specifically, there is inductively coupled to the horizontal output transformer windings H2 and H3 a third winding I29, which comprises portions I and I3I, having a tap I32 therebetween.

Portion I3I is included in the anode circuit of tube I35, which may be traced from the anode through conductor I45, winding I 3| tap I32, and conductor I44, to a time constant network comprising a parallel combination of a resistor I40 and a capacitor I38, connected between conductor I44 and ground. The line-frequency pulses which are developed across winding I3I during retrace intervals are of such a polarity as to render tube I conductive, and this tube plate-rectifies into the time-constant network I38, I40.

network comprising resistor 43, resistor I36, and

capacitor I37. Resistor I33 isolates capacitor 8. I31 from the video channel, including the elements 53, 54. It will be noted that the grid of tube I35 is D. C. coupledto the detector, so that the D. C. component of the video signal is not lost. Resistor I35 and capacitor I31 provide a filter which cuts off at approximately 150 kilocycles, thereby discriminating against noise and interference. It is desirable that the filter comprising the elements I36 and I3! have a cutoff characteristic such' that the synchronizing pulse components derived by the video detector andappearing at the output thereof are applied with substantially full amplitude to the input of tube I35, although distortion as to the wave forms of these pulses is desired. In other words, the network I36, I31 should discriminate against high frequency noise and in doing so will somewhat distort the wave shapes. The object of the filter I36, I3! is to apply these pulses to the grid of tube I35 with the optimum signal-to-noise ratio. The fundamental of these pulses being on the order of 15,000 cycles, a filter having high frequency cutoff characteristics of 150 kilocycles The elements I35, I38, I40 are com- V prised in the means for developing a negative has been found entirely satisfactory. The output of such a filter contains the necessary information concerning the peak height of the synchronizing signals. Any increase in band width, over that required to indicate such height would reduce the discrimination against noise and serve no useful purpose. As pointed out in the copending patent application of Francis A. Wissel and Norman W. Parker, Serial No. 102,176, filed in the United States Patent Office on June 30, 1949, and assigned to the same assignee as the present application and invention, resistor I36 also provides; grid circuit clipping of large impulse noise pulses of positive polarity. This series grid resistor I36 is very large compared to the grid-to-cathode resistance of tube I35 when grid current fiows so that the input signal to tube I 35 is limited to the voltage level of the grounded cathode. When noise pulses tend to cause the grid of tube I35 to become positive with respect to the cathode, the grid-to-cathode resistance drops to a negligible value with respect to resistor I36 and the grid current flow in resistor I36 develops a bias which opposes the positive noise pulses.

The input admittance of tube I35 is neutralized by a variable capacitor I which couples the plate and grid circuits of tube I35 in such a way that the current passing through it is of proper amplitude and phase to neutralize the effect of the current flowing between the plate and grid circuits of tube I35 via the grid-plate capacitance. The voltage at the end of winding portion I30, connected to capacitor I 56, is in phase opposition to the voltage at the end of winding portion I3I connected to the plate of tube I35.

The voltage across the neutralizing inductance I30 causes the gridof tube I35, connected to capacitor I50, to receive a current which neutralizes energy transfer through the tube capacitance as explained in Terman, Radio Engineering, pages 367 et seq., Third Edition, 1947, McGraw-Hill Book C0,, Inc.,- New York, New York.

Capacitor I3! also aids in minimizing the critical character of the neutralization.

As stated above, the control pulses are applied to the input circuit of tube I35 to develop an amplified A. G. C. potential. This is accomplished by plate rectification of the line-frequency pulses of positive polarity, applied to the plate circuit of tube I 35 These pulses are developed during retrace intervals, in. winding. I3I, so included in the horizontal output transformer.

Tube I35 is periodically keyed into conductivity to develop, by plate-rectification, across the time-constant network I38, Mil an amplified A. G. C. potential. The pulses applied to the anode of tube 535 being constant, its platecurrent pulses, of. negative polarity, are a function of the amplitude of the grid voltage peaks, which in turn are a function of the line-frequency synchronizing signal amplitude, the latter being a function of video signaL output level at the detector. As indicated in the above-mentioned copending Wisseland Parker patent application, this type of A. G. C. system is particularly insensitive to-noise in that the amplified. A. G. C. potential is. developed during phase coincidence of the pulses applied to grid and plate of tube I35, which pulses occupy but a very small total time as compared to thev duration of one line, for example. During the remaining major time portions, the system is entirely immune from noise interference.

Referring now to the time constant network I38, I40, it should possessthe following desirable characteristics:

Resistor I IFJv has, a value many times that of the plate resistance of tube I35. in order to pro.- vide good gain; capacitor I38. has a value which gives a compromisev between two requirements, namely: (1) The capacitor should be large enough to insure high output voltage from the rectifier I35, and (2) the capacitor should be small. enough to. provide a fast time constant so that the negative voltage at point Z will be ad.- justecl in value every few lines, which will insure a maximum immunity to noise.

The automatic gain control circuit in accordance with the invention comprises the following principal components:

First, a source of negative potential having an amplitude which increases in accordance with the video output level of detector 48;

Second, a source of-positive potential;

Third, a resistor network to one end of which the positive potential is applied and to the other end of which the negative potential is applied,, whereby the ground-potential point on said network is shifted toward the first-mentioned end as the video output level increases, and whereby distinct A. G. C. potentials are developed at spaced points on said network;

Fourth, first and second coupling and filter networks independently connected between said spaced points and the I. F. and R. F. controlled stages, respectively; and

Fifth, a clamping diode which maintains substantially at ground potential that point to which the first coupling network is connected, until said diode becomes nonconductive, at which time said point goes negative.

The fixed source of positive potential need not be shown in detail. Its positive is indicated by the symbol +B, at one terminal of resistor I80.

The inventive combination comprises a resistor network I88, I8I, I82, arranged in series. The positive source is. connected to resistor I88 at point X. The negative source is connected to resistor I82 at pointZ. Since. one end (X) of this. network I30, I8I, I82 isinitially positive, and the. other end Z is initially negative, there would bev a ground-potential point in the network to the right of Y if for the moment the operation of diode I43 be neglected. This point would. be;

10: shifted away from Z and toward X as the video output level atthe detector increases.

A. G. C. potential is applied'to the control electrode of the R. F. input tube 2 bya second coupling or filter network comprising a shunt capacitor I83, a series resistor I88, a shunt capacitor I84, 2. series resistor I 86 and a shunt capacitor I85, connected between point Y and that control electrode.

A. G. C. potential is applied to the control electrodes of the I. F. amplifier tubes I2 and I8 by a first coupling network. The'junction of resistors I81, I82 is connected to the grid resistor I3. of tube I8, a shunt filter capacitor I54 being provided. That. junction isalso connected to the grid resistor I4 of the tube I2, through a filter comprising a series resistor I 55 and a shunt capacitor I5.

The first amplified-A. G. C. potential appearing at point J, the junction of resistors I8I and I82, is applied to the I. F. stages. The second amplified A. G. C. potential, appearing at point Y, the junction of resistor I 88 and the anode of diode I43, is applied to the R. F. stage. Curves- A, B, and C of Fig. 3 show A. G. C. potential. applied to I. F. stages, A. G. C. potential applied to R. F. stages and detector output level, respectively, as ordinates, plotted against input signals as abscissae. It will be noted that a portion of the available A. G. C. potential is first applied to the I. F. stages and that, as input signal level increases, more potential is applied to the I. F. stages until the input level becomes such that A. G. C. potential is applied. at a slowrate: of increase to the R. F. stage. Finally full A. G- C. potential is applied at substantially the same rate of increase to both stages.

As stated above, as the video detector output increases, the assumed ground-potential point innetwork Iii-I, I82 tends to be shifted to the left. Initially, when the output level is such that tube I35 becomes conductive, the assumed ground potential point is located between. points Y and J, sothat a portion of the available-negative A. G. C. potential is; applied to'the I. F. stagesas soonas tube I35 becomes conductive. This potential be-- comes increasingly negative as the output level. increases.

Point Y is to the left ofthe assumed ground p0- tentia-l point and would be at a substantial positive potential at the initiation of the application of A. G. C. potential to the I. F. stages, were it. notior the action of diode I43. This diode and its cathode resistor I48 are. connected between point Y and ground.

The diode is provided with a cathode resistor I48, the high potential side of which is connected through a resistor I41 to the positive terminal of a source of potential (not shown), the purpose of. which is'to cause to flow in resistor I48 a current of positive polarity and a voltage drop of such. magnitude as to overcome the contact potential of the diode I43. The term contact potential is well-known to those skilled in the art and this. phenomenon is discussed at pages 8 and 9 of Reichs Theory and Applicationsof Electron Tubes, second edition, 1944, McGraw-Hill Book (30;, Inc., New York, New York; The provision being made for overcoming contact potential, the following discussion will be postulated onthe assumption that the cathode of diodev I43 is at ground potential. I

The diode is conductive, so long as the result- 11 diode is positive; i. e., until the video detector output level is such that the ground-potential point in network I80, I8I, I82 is shifted to the left of point Y and on to resistor I80. The diode therefore holds point Y at substantially ground potential until the ground potential point that would be produced by the action of the above-mentioned two opposed voltages is shifted to the left of point Y. When the video output level becomes sufficiently great to render the diode nonconductive, point Y'then goes negative and A. G. C. potential is applied to R. F. tube 2. It will be noted that the source +B, the network I80, IBI, I82, diode I43 and the negative source I35, I40, I33,

automatically accomplish the required delay between the initial applications of A. G. C. poten tial to the R. F. and I. F. stages.

Referring now specifically to Fig. 2 for aid in describing the operation of the invention, in accordance with another approach in exposition,

' the potentials of points'X, Y, J, and Z are illustrated by corresponding symbols. Let it be assumed that the carrier signal level is increasing. Point X is always positive and is maintained so by a source of positive potential, +3. Point Z is at a negative potential as represented by the symbol Z1. Curve C of Fig. 2 illustrates what the voltage conditions would be at points X, Y, and Z if it were not for the action of diode I43. Were it not for that diode, point Y would be at a positive potential with respect to ground. It will be obvious from an inspection of curve C that, as the video detector output level increases, the point at which curve C intercepts the ground potential linewill continue to shift to the left. This point is the assumed ground potential point mentioned above. The curve A illustrates the effect of the diode action in clamping point Y substantially at ground potential. From an inspection of curve A, it will be seen that as soon as tube I35 becomes conductive pointZ is at a negative potential, as illustrated by Z1, and A. G. C. voltage V1 is applied to the intermediate frequency stages, the potential at point J being indicated by J1. Under these conditions point Y is substantially at ground potential, as indicated by the symbol Y1, and no A. G. C. voltage is applied to the R. F. stage.

Curve B illustrates the events which would occur in the absence of the source of positive potential connected to point X. If X (the terminal of resistor I80 remote from point Y) were at ground potential, A. G. C. potentials, as indicated by curve B, would simultaneously begin to be applied at the same rate of increase to the R. F.

ing diode, so that a negative A. G. C. potential V2 is now applied to the R. F. stage. Thus it will be seen that the positive voltage source causes the application of A. G. C. potential to the R. stage to be delayed. It will also be seen that the action of the clamping diode is suchas to prevent A.. G. C. potential from being applied to the R. F.

stage until the I. F. stages have attained a rela tively high signal level.

It would be possible to place point X at ground potential and to swing points Y and J negative at thesame rate. This condition is represented by curve B. Such an arrangement would notaccomplish the objects of this invention, which requires this sequence of events: partial application of A. G. C. potential to the I. F. stages, delayed application of A. G. C. potential to the R. F. stage, finally increasing A. G. C. potential to both I. F. and R. F. stages. Curve B shows that these objects require the use of means to prevent point Y from going below ground potential until the desired delay is accomplished.

"The required delay is produced by the positive source. It would be possible, as shown by curve C, to omit the diode. But the tolerances prevailing in television receiver manufacturing practice would occasion so much fortuitous shifting of the ground point that adjustment would be impracticable. Therefore the combination of the positive source and the diode is used to clamp point Y at ground potential so that the negative swing is first about point Y and finally about point X as a reference when signal level increases.

It will be noted that tube I35 plate-rectifies into network I 40, I38, shunted by the series load of resistors I8I, I82 and diode I43. When diode I43 become nonconductive, the load impedance in the plate circuit of the tube I35 suddenly increases, due to the effective removal of the shunt. This factor tends further to increase the A. G. C. potential applied to the I. F. stages as the application of gain control potential to R. F. stage is initiated.

Curve A represents the potential levels in network I80, I8I, I82, at points from X to Y, under a low input signal level condition. Curve B represents the potential levels under a high input signal condition. These curves are drawn for purposes of illustration and are not proportioned to the resistance values, illustrative parameters being given in-the'following paragraph.

The following circuit parameters have been embodiment of the invention:

Capacitor II 100 mmf, Capacitor I 0.01 mfd, Resistor I4 10,000 ohms Resistor i3 8200 ohms Resistor 68 ohms Tubes I2, I8, 23 Type 6AU6 Capacitors Ii, 22, 2'? 100 f,

. Resistor 20 68 ohms Capacitor 25-; 5000 mmf. Resistor I56 5600 ohms Resistor I5! 10,000 ohms Resistor 29 100 ohms Capacitor 30 5000 mmf. Capacitor 5000 mmf. Resistors 38, 32, 35, 35, 33 100 ohms Capacitors 30, 31, 30, BL"; e- 500 mmf. Capacitor 46 '100 mmf. Tube 28; Type 6AG5 Tube is Type 6AL5 Resistor 30 5600 ohms Capacitor I58 5000 mmf. Capacitor 50 5 mmf. Capacitor 54 22,000 ohms Tube 55 Type 6AG5 Resistor 55 39,000 ohms Resistor BI 22,000 ohms Capacitor 5! 4 mfd.

Capacitor 32 0.05 mfd.

Resistor I2 3300 ohms Capacitor 67 0.1 mfd. Resistor 65 4700 ohms Resistor 68 820,000 ohms Resistor 69 270,000 ohms Resistor I 47,000 ohms Tube 63 Type BP4 Capacitor 83 -240 mmf. Capacitor I9 0.002 mfd. Resistor 81 820,000 ohms Capacitor 93 0.25 mfd. Resistor 92 8200 ohms Capacitor 94 0.02 mfd.

Resistor 9| 150,000 ohms Resistor 88 150,000 ohms Tube sections 80, 89 Type 6S'N7GT Resistor 81 2.7 megohms Capacitor 95 0.05 mfd. Resistor 97 268,000 ohms, variable Resistor 90 100,000 ohms Resistor I60 10,000 ohms Capacitor 99 200 mmf. Capacitor 82 5mmf. Resistor 8| 560,000 ohms Resistor 85 150,000 ohms Capacitor I03 0.01 mfd. Resistor I04 22,000 ohms Resistor I05 120,000 ohms Resistor I10 1000 ohms Capacitor 84 2000 mmf. Capacitor I06 390 mmf. Capacitor I63 10-160 mmf. Resistor I01 1 megohm Resistor I64 47 ohms Tube I08 Type 6BG6G Capacitor IIO 5mfd. Resistor I09 82 ohms Capacitor I28 0.05 mfd- Capacitor II9 0.1 mfd. Capacitor I24 0.05 mfd. Resistor I2I 1000 ohms Capacitor I66 0.5 mfd. Tube II8 Type 6W4GT Capacitor IIB 0.5 mfd. Tube I43 Half of type 6AL5 Tube I Half of type 12AU7 Resistor I48 180 ohms Resistor I41 150,000 ohms Resistor I 100,000 ohms Capacitor I38 0.015 mfd. Capacitor I50 2.5 mmf. Tube 66 Half of type 12AU7 Resistor I36 27,000 ohms Capacitor I31 56 mmf. Resistor I 10,000 ohms Capacitor I54 0.01 mfd. Capacitors 4|, 43; 5000 mmf. Resistor 42 100 ohms Tube 2 6AB4 Resistor I 80 5.6 megohms Resistor I8I 33,000 ohms Resistor I82 270,000 ohms Resistor I 1 megohm Capacitor I83 5000 mmf. Resistor I 88 1000 ohms Capacitor I85 1000 mmf. Resistor I86 220,000 ohms While there has been shown and described what is at present considered to be the preferred embodiment of the present invention, it will be obvious to those skilled in the art that various modifications and substitutions of equivalents may be made without departing from the true scope of the invention as defined by the appended claim. It will, of course, be understood that this invention is also applicable to radio receivers.

I claim:

An automatic gain control system, for use with a television receiver of the type including a gaincontrolled video channel having radio frequency and intermediate frequency stages, a video detector and a line-frequency deflection system having a sweep transformer and providing a source of line-frequency pulses, comprising the combination of means keyed by the line-frequency signal output of said detector for rectifying line-frequency pulses to provide a source of negative potential having an amplitude which varies in accordance with said output, said means comprising an auxiliary winding on said transformer and an electron tube, said tube having a cathode-control electrode circuit coupled to said detector and an anode-cathode circuit in series with said auxiliary winding, an anode load circuit comprising a first resistor and a parallel capacitor connected in series with said winding and said cathode, said first resistor and parallel capacitor having a time-constant product of approximately twenty-three lines, asource of positive potential having a positive terminal, a second resistor and third and fourth resistors connected in series and in a direct current path with said terminal and said winding and said anode, a diode coupled between the junction of the second and third resistors and said cathode and having its own anode connected to said junction for holding said junction substantially at ground potential until the output of said detector reaches such a level that said junction goes negative, means for coupling said junction to said radio frequency stage to apply an A. G. C. potential thereto, and means for coupling the junction of said third and fourth resistors to said intermediate frequency stage to apply an A. G. C. potential thereto, the lastnamed junction being located between the firstmentioned junction and the source of negative potential, whereby a part of the available negative A. G. C. potential is first applied to the I. F. stages alone, and, after an increase in carrier signal level, A. G. C. potentials not affected by said diode are applied to both R. F. and I. F.

stages.

HARLAND A. BASS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,117,664 Holst May 17, 1938 2,159,803 Klotz May 23, 1939 2,164,979 Wilhelm July 4, 1939 2,171,657 Klotz Sept. 5, 1939 2,199,169 Dome Apr. 30, 1940 2,200,049 Van Loon May 7, 1940 2,223,982 Bedford Dec. 3, 1940 2,300,942 Lewis Nov, 3, 1942 2,303,909 Blumlein Dec. 1, 1942 2,307,375 Blumlein et al Jan. 5, 1943 2,538,519 Holst Jan. 16, 1951 FOREIGN PATENTS Number Country Date 845,897 France Sept. 4, 1939 873,623 France July 15, 1942 

